routefilters

Routefilters are policy constructs used in computer networks to control which routes are learned, accepted, or advertised by routers. They are commonly applied in inter-domain routing using the Border Gateway Protocol (BGP), but similar filtering concepts also exist within interior routing protocols and during redistribution between protocols.

Filters are implemented with a combination of mechanisms such as prefix filters (prefix-lists or prefix-based access control lists) that match destination prefixes, AS-path filters that match the sequence of autonomous systems, and route maps or policy statements that attach actions (permit or deny) and may consider other criteria such as next hop, community attributes, or MEDs.

Filters can be applied to inbound or outbound updates, to routes learned from peers or to locally originated routes. They are used to enforce business policies, prevent routing leaks, control the growth of routing tables, implement traffic engineering, and manage security. By constraining which routes can enter or leave a router’s routing table, routefilters help maintain predictable routing behavior and reduce the risk of unintended route propagation.

Vendor-specific syntax varies, but common approaches include prefix-lists, AS-path filters, route-maps (or policy statements), and community-based filters. A typical best practice is to configure a default-deny policy with explicit permits and to log filter matches for auditing and troubleshooting. Considerations when deploying routefilters include testing changes in a lab, staged rollouts, thorough documentation, and vigilant monitoring to avoid unintended loss of reachability.

See also: BGP, routing policy, prefix-list, AS-path filter, community, distribute-list.